One of the great things about being Dr. Know (aside from the lovely tiara, and the power to turn milk sour with a glance) is the fact that, on the rare occasion I can't find the answer to a question, the readers will. And that's good, because this one is surprisingly difficult to get to the bottom of.
First things first: Metro will accept your motor oil in opaque jugs if you take it to the transfer station yourself—it's only when you put it on the curb that it's snubbed.
According to Metro's Jim Middaugh, the transparent-jug rule is part of the agreement between the city of Portland and the private trash haulers. A city employee then told me, unofficially, that the haulers want to be sure that it's really oil in the jug. But since they turn over that jug, unopened, to the same transfer station that accepts the opaque jugs anyway, why should anyone care?
Here's another interesting tidbit: In Seattle, they accept opaque jugs curbside! But they have to be one-gallon jugs, which still rules out the one-quart jugs new oil usually comes in. Suspicious, no?
Since neither of the two private haulers I called with this question got back to me, I figure I'll just make something up and wait for the phone to ring:
It's a conspiracy. The garbage company doesn't want to deal with your oil, so they make it tough to recycle on purpose. Also, most garbagemen are werewolves. Anybody wants to correct me, drop me a line.